选取DropDownList的值，总是第一个值？

[hagain]

DDL1 的项是省份；DDL2的项是城市。现在选择DDL1不同的省份，DDL2显示相应的城市。但每次选择后，刷新页面后所DDL1显示的都是第一个值，不会改变的？
private void Page_Load(object sender, System.EventArgs e)
{
if(!this.IsPostBack )
{ OleDbConnection conn=DB.conn(); string sql="select sheng from city group by sheng";
OleDbDataAdapter da=new OleDbDataAdapter();
OleDbCommand cmd=new OleDbCommand(sql,conn);
conn.Open();
da.SelectCommand =cmd;
DataSet ds=new DataSet();
da.Fill (ds);
this.ddlSheng.DataSource=ds.Tables[0];
this.ddlSheng.DataMember = ds.Tables[0].Columns[0].ToString();
this.ddlSheng.DataValueField = ds.Tables[0].Columns[0].ToString();
this.ddlSheng.DataBind();
ds.Clear();
conn.Close();
}
private void Button1_Click(object sender, System.EventArgs e)
{
this.ddlCity.Items.Clear();
OleDbConnection conn=DB.conn(); //列出DDL1相应的城市
string sheng=this.ddlSheng.SelectedValue ;
this.Label1.Text =""+sheng+"";
string sql="select city from city where sheng='"+sheng+"'";
OleDbDataAdapter da=new OleDbDataAdapter();
OleDbCommand cmd=new OleDbCommand(sql,conn);

conn.Open();
da.SelectCommand =cmd;
DataSet ds=new DataSet();
da.Fill(ds);
this.ddlCity.DataSource=ds.Tables[0];
this.ddlCity.DataMember = ds.Tables[0].Columns[0].ToString();
this.ddlCity.DataValueField = ds.Tables[0].Columns[0].ToString();
this.ddlCity.DataBind();
ds.Clear();
conn.Close();
}

}

[wang520d]

aotopostback = false;

[chy710]

<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">
</asp:DropDownList>


protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
// ....
}

[Aden]

string sheng=this.ddlSheng.SelectedValue ;

改成下面：
string sheng　= Request.Form["ddlSheng"];

[mqc507]

在DDL1_SelectIndexChanged()中写：

ListItem LI=(ListItem)DDL1.Items.FindByText("页面传递的变量");//如果你选择城市1,应 有 this.aspx?city="+DDL1.SelectItem.Text+"吧 ？那这个“页面传递的变量”就是“city”
if(LI!=null)
{
LI.Selected=true;
}

[mqc507]

在DDL1_SelectIndexChanged()中写：

页面传递的变量= request.QueryString["city"];
ListItem LI=(ListItem)DDL1.Items.FindByText("页面传递的变量");//如果你选择城市1,应 有 this.aspx?city="+DDL1.SelectItem.Text+"吧 ？那这个“页面传递的变量”就是“city”
if(LI!=null)
{
LI.Selected=true;
}

[youthkin]

autopostback属性设为true没有？